Case hardening steel, carburized component, and manufacturing method of case hardening steel

ABSTRACT

A case hardening steel includes, as a chemical composition, by mass %, C: 0.10% to 0.30%, Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S: 0.003% to 0.020%, Cr: 0.40% to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to 0.20%, Bi: 0.0001% to 0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0% to 0.50%, B: 0% to 0.0050%, Nb: 0% or more and less than 0.040%, P: limited to 0.050% or less, N: limited to 0.0060% or less, O: limited to 0.0025% or less, and a remainder including an iron and impurities, and satisfies Ti/S≥6.0, in which, in a longitudinal section, a maximum diameter of Ti-based precipitates predicted by extreme value statistics under a condition that an inspection standard area is 100 mm2, a number of inspections is 16 visual fields, and an area where prediction is performed is 30,000 mm2, is 40 μm or less.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a case hardening steel, a carburizedcomponent, and a manufacturing method of a case hardening steel, andparticularly to a case hardening steel which is excellent in coarsegrain prevention properties and fatigue properties during carburizing, amanufacturing method thereof, and a carburized component which isobtained from the case hardening steel.

Priority is claimed on Japanese Patent Application No. 2015-256254,filed on Dec. 28, 2015, the content of which is incorporated herein byreference.

RELATED ART

Carburized components such as gears, bearing components, rollingcomponents, shafts, and constant-velocity joint components are typicallymanufactured by a method of forging medium carbon alloy steel formachine structural use specified in, for example, JIS G 4052, JIS G4104, JIS G 4105, and JIS G 4106, machining the forged steel into apredetermined shape, and performing carburizing and quenching thereon.

During the forging performed when the carburized component ismanufactured, cold forging (including rolling) or hot forging isperformed. In cold forging, the surface skin and dimensional accuracy ofa product are good, manufacturing costs are lower than in hot forging,and good yield is achieved. For this reason, in recent years, as theforging in the manufacturing of a carburized component, cold forging hasbeen increasingly performed instead of hot forging. As a result, thenumber of carburized components manufactured by performing carburizingand quenching after cold forging has been remarkably increased in recentyears.

However, as a main object of carburized components manufactured byperforming carburizing and quenching after cold forging is to reduceheat treatment strain. For example, when a shaft is bent due to heattreatment strain, the function of the shaft is impaired. In addition,high heat treatment strain in gears and constant-velocity jointcomponent causes noise and vibration.

The greatest cause of heat treatment strain in carburized components iscoarse grains generated during carburizing. In the related art, in orderto suppress the generation of coarse grains, annealing is performedbefore carburizing and quenching and after cold forging. However, inrecent years, from the viewpoint of cost reduction, there is a strongdemand for the omission of annealing. Therefore, there is a strongdemand for steel in which coarse grains are not formed duringcarburizing even when annealing is omitted before carburizing andquenching.

On the other hand, among gears, bearing components, and rollingcomponents, an object of the bearing components and rolling componentsthat receive high contact pressure is to sufficiently ensure fatigueproperties such as rolling fatigue properties. In such components,internal origin type damage is more frequently seen than surface origintype damage, and thus high depth carburizing is performed in order tosuppress damage slightly inward of the surface where shear stress islikely to be maximized. However, since this high depth carburizingtypically takes a long period of time of a dozen or so hours to severaltens of hours, from the viewpoint of energy saving, a reduction in thecarburizing time is required. In order to reduce the carburizing time,an increase in the carburizing temperature is effective. That is, thecarburizing time can be reduced by setting the carburizing temperature,which is about 930° C. in typical carburizing, to be in a temperaturerange of 990° C. to 1090° C. However, when high temperature carburizingis performed in a temperature range of 990° C. to 1090° C. in order toreduce the carburizing time, coarse grains are generated, and there maybe cases where fatigue properties such as rolling fatigue propertiesneeded for carburized components are not sufficiently obtained.Therefore, there is a demand for case hardening steel which does notcause the generation of coarse grains even when high temperaturecarburizing is performed and is thus suitable for high temperaturecarburizing.

In addition, gears, bearing components, and rolling components whichreceive high contact pressure are generally large components. Such largecomponents are typically manufactured by hot forging a steel bar,performing a heat treatment such as normalizing as necessary, andperforming machining, carburizing and quenching, tempering, andpolishing as necessary. In order to suppress the generation of coarsegrains during carburizing, a hot forged member after the hot forgingneeds to be an appropriate material capable of suppressing coarse grainsduring carburizing. For this, it is necessary to use steel capable ofsuppressing the generation of coarse grains during carburizing as thematerial of the steel bar.

Patent Document 1 discloses a case hardening steel which contains Ti:0.05% to 0.2%, S: 0.001% to 0.15%, and N: limited to less than 0.0051%,in which the amount of AIN precipitated after hot rolling is limited to0.01% or less and coarse grain prevention properties and fatigueproperties during carburizing are excellent.

In addition, Patent Document 2 discloses a case hardening steel whichcontains Ti: 0.03% to 0.30%, S: 0.010% to 0.10%, N: limited to 0.020% orless, in which the number density of Ti-based sulfides is specified.

However, in the case hardening steels disclosed in Patent Documents 1and 2, a reduction in pinning force due to coarsening of precipitatesused for pinning of grains and the balance between Ti and S contents arenot considered, and there is a possibility that Ti-based precipitatesused for the pinning effect may be insufficient. As a result, in thecase hardening steels disclosed in Patent Documents 1 and 2, in a casewhere switching from hot forging to cold forging, annealing after thecold forging is omitted, or high temperature carburizing and the likeare performed, there is a possibility that the coarse grain preventionproperties may be insufficient. In addition, gears and shafts, which aremain applications of case hardening steel, require not only a reductionin thermal strain but also a further improvement of fatigue strengthproperties.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Patent No. 4448456

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2007-31787

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made taking the foregoing circumstancesinto consideration, and an object thereof is to provide a case hardeningsteel having excellent coarse grain prevention properties duringcarburizing, a carburized component, and a manufacturing method of acase hardening steel. In a case of excellent coarse grain preventionproperties during carburizing, heat treatment strain due to carburizingand quenching can be suppressed even when annealing before thecarburizing is omitted, and excellent fatigue properties can be obtainedafter the carburizing and quenching.

Means for Solving the Problem

(1) According to an aspect of the present invention, a case hardeningsteel includes, as a chemical composition, by mass %: C: 0.10% to 0.30%,Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S: 0.003% to 0.020%, Cr: 0.40%to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to 0.20%, Bi: 0.0001% to0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0% to 0.50%, B: 0% to0.0050%, Nb: 0% or more and less than 0.040%, P: limited to 0.050% orless, N: limited to 0.0060% or less, O: limited to 0.0025% or less, anda remainder including an iron and impurities, in which an Expression (a)is satisfied, and in a longitudinal section, a maximum diameter ofTi-based precipitates predicted by extreme value statistics under acondition that an inspection standard area is 100 mm², a number ofinspections is 16 visual fields, and an area where prediction isperformed is 30,000 mm², is 40 μm or less,

Ti/S≥6.0   Expression (a)

where Ti in the Expression (a) represents a Ti content by mass %, and Sin the Expression (a) represents a S content by mass %.

(2) The case hardening steel according to (1) may include, as thechemical composition, by mass %, one or more selected from the groupconsisting of: Mo: 0.02% to 1.50%, Ni: 0.10% to 3.50%, V: 0.02% to0.50%, B: 0.0002% to 0.0050%, and Nb: more than 0% and less than 0.040%.

(3) In the case hardening steel according to (1) or (2), ametallographic structure may contain a bainite, and a structure fractionof the bainite may be 30% or less.

(4) In the case hardening steel according to any one of (1) to (3), themetallographic structure may contain a ferrite, and a grain size numberof the ferrite may be No. 8 to No. 11 specified in JIS G 0552.

(5) According to another aspect of the present invention, a carburizedcomponent includes: the case hardening steel according to any one of (1)to (4).

(6) According to still another aspect of the present invention, amanufacturing method of a case hardening steel includes: a heatingprocess of heating steel including, as a chemical composition, by mass,C: 0.10% to 0.30%, Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S: 0.003% to0.020%, Cr: 0.40% to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to 0.20%,Bi: 0.0001% to 0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0% to0.50%, B: 0% to 0.0050%, Nb: 0% or more and less than 0.040%, P: limitedto 0.050% or less, N: limited to 0.0060% or less, O: limited to 0.0025%or less, and a remainder including an iron and impurities, and satisfiesan Expression (b), at a temperature of 1150° C. or higher for a holdingtime of ten minutes or longer; and a hot rolling process of hot rollingthe steel into a wire rod or a steel bar,

Ti/S≥6.0   Expression (b)

where Ti in the Expression (b) represents a Ti content (mass %), and Sin the Expression (b) represents a S content (mass %).

(7) The manufacturing method of a case hardening steel according to (6)may further include: a cooling process of slow cooling the wire rod orthe steel bar at a cooling rate of 1.00° C./s or less in a temperaturerange of 800° C. to 500° C. after the hot rolling process.

(8) In the manufacturing method of a case hardening steel according to(6) or (7), in the hot rolling process, a finish temperature may be setto 840° C. to 1000° C.

Effects of the Invention

The case hardening steel according to the aspect of the presentinvention has a predetermined chemical composition, and the maximumdiameter of the Ti-based precipitates is controlled to be in apredetermined range, thereby achieving excellent coarse grain preventionproperties during carburizing. Therefore, with the case hardening steelaccording to the aspect of the present invention, heat treatment straindue to carburizing and quenching can be suppressed, and excellentfatigue properties are obtained after the carburizing and quenching. Inaddition, the carburized component according to the aspect of thepresent invention has less heat treatment strain and has excellentfatigue properties.

With the manufacturing method of a case hardening steel according to theaspect of the present invention, the case hardening steel which hasexcellent coarse grain prevention properties during carburizing can bemanufactured. The case hardening steel obtained by the manufacturingmethod can suppress heat treatment strain due to carburizing andquenching and achieve excellent fatigue properties after the carburizingand quenching.

EMBODIMENTS OF THE INVENTION

In order to achieve the above objects, the inventors conducted intensivestudies. As a result, the following findings (i) to (v) were obtained.

(i) In the related art, there may be cases where crystallization of MnS,which is the origin of bending fatigue fracture, occurs duringcarburizing performed on case hardening steel, and sufficient fatigueproperties cannot be obtained after carburizing and quenching performedon the case hardening steel. Contrary to this, by optimizing therelationship between the S content and the Ti content in case hardeningsteel (Ti/S≥6.0), which is not considered in the related art, fineTi-based carbosulfide can be formed instead of MnS, which is stretchedin a rolling direction in which bending fatigue properties aredeteriorated and is coarsened during carburizing performed on casehardening steel. Since fine Ti-based carbosulfide is formed instead ofcoarse MnS, excellent fatigue properties are obtained after carburizingand quenching performed on the case hardening steel.

(ii) In order to prevent coarsening of grains during carburizingperformed on case hardening steel, it is effective to use Ti-basedprecipitates mainly containing TiC and TiCS finely precipitated duringcarburizing, instead of using AIN and NbN as pinning particles.Furthermore, by including a small amount of Bi in the case hardeningsteel, growth and coarsening of the Ti-based precipitates during thecarburizing are suppressed, and thus coarse grain prevention propertiesare further improved.

In order to stably exhibit the pinning effect of the Ti-basedprecipitates during the carburizing performed on the case hardeningsteel, it is necessary that Ti-based precipitates finely precipitate insteel which is cooled after being hot rolled in a manufacturing processof the case hardening steel. For this, it is necessary to cause Ti-basedprecipitates to precipitate at phase interfaces during diffusiontransformation from austenite in the cooling process after the hotrolling. When bainite is formed in the hot rolled structure, it isdifficult for the Ti-based precipitates to precipitate at phaseinterfaces. Therefore, it is preferable to suppress the formation ofbainite in case hardening steel as much as possible.

In order to cause Ti-based precipitates to finely precipitate in steelafter being hot rolled and cooled, it is effective to optimize the hotrolling conditions. That is, it is preferable that Ti-based precipitatesare first solid-solubilized in a matrix by setting the heatingtemperature during the hot rolling to a high temperature and theprecipitation temperature range of the Ti-based precipitates after thehot rolling is set such that slow cooling is achieved. Through theheating, rolling, and cooling, the formation of bainite can besuppressed, and a large amount of Ti-based precipitates can be formedand finely dispersed.

(iii) By causing Nb carbonitride mainly containing NbC to finelyprecipitate during the carburizing performed on the case hardening steelin combination with the Ti-based precipitates, the coarse grainprevention properties are further improved. In order to stably exhibitthe pinning effect of the Nb carbonitride during the carburizingperformed on the case hardening steel, it is necessary that the Nbcarbonitride finely precipitates in steel after being hot rolled andcooled in the manufacturing process of the case hardening steel. Forthis, similarly to the Ti-based precipitates, it is necessary to causethe Nb carbonitride to precipitate at phase interfaces during diffusiontransformation from austenite in the cooling process after the hotrolling.

In order to cause the Nb carbonitride to finely precipitate in the steelafter being hot rolled and cooled, it is preferable that Nb carbonitrideis first solid-solubilized in a matrix by setting the heatingtemperature during the hot rolling to a high temperature and theprecipitation temperature range of the Nb carbonitride is set such thatslow cooling is achieved. Through the heating, rolling, and cooling, alarge amount of Nb carbonitride can be finely dispersed. When bainite isformed in the hot rolled structure, it is difficult for the Nbcarbonitride to precipitate at phase interfaces. Therefore, it ispreferable to suppress the formation of bainite as much as possible.

(iv) When ferrite grains contained in the steel after being hot rolledand cooled are excessively fine, coarse grains tend to be generatedduring the carburizing performed on the case hardening steel. The grainsize of the ferrite grains in the steel after being hot rolled andcooled can be optimized by controlling the finish rolling temperature.

(v) In a carburized component manufactured by performing carburizing andquenching on case hardening steel containing Ti, Ti-based precipitatesbecome the origin of fatigue fracture, so that fatigue properties,particularly rolling fatigue properties tend to be insufficient. It ispossible to improve the fatigue properties by reducing the N content inthe chemical composition of the case hardening steel, setting theheating temperature during hot rolling to a high temperature, andreducing the maximum size of Ti precipitates.

The present invention has been made based on the above-described novelfindings.

Hereinafter, case hardening steel according to an embodiment of thepresent invention (case hardening steel according to this embodiment), acarburized component according to the embodiment of the presentinvention (a carburized component according to this embodiment), and amanufacturing method thereof will be described in detail.

First, the chemical composition of the case hardening steel according tothis embodiment will be described. Unless otherwise specified, “%” ofthe amount of each element means “mass %”.

(C: 0.10% to 0.30%)

C is an element effective in improving the strength of steel. When the Ccontent is less than 0.10%, a preferable tensile strength (for example,about 900 MPa) cannot be secured after carburizing, quenching andtempering. On the other hand, when the C content exceeds 0.30%, thesteel becomes hard, the cold workability deteriorates, and the toughnessof a core portion after the carburizing and quenching deteriorates.Therefore, it is necessary to set the C content to be in a range of0.10% to 0.30%.

(Si: 0.02% to 1.50%)

Si is an element effective in deoxidizing steel. In addition, Si is anelement effective in imparting necessary strength and hardenability tosteel and improving the temper softening resistance of the steel. Whenthe Si content is less than 0.02%, the effect is not sufficientlyobtained. On the other hand, when the Si content exceeds 1.50%, thehardness of the steel increases and the cold forgeability deteriorates.For the above reasons, it is necessary to set the Si content to be in arange of 0.02% to 1.50%.

In a case where the case hardening steel is subjected to cold working, asuitable range of the Si content is 0.02% to 0.30%. In particular, in acase where cold forgeability is considered as being important, it ismore desirable that the Si content is set to be in a range of 0.02% to0.15%.

In addition, Si is an element effective in increasing the grain boundarystrength, and in a case where case hardening steel is used as thematerial of a carburized component such as a bearing component and arolling component, Si is an element effective in increasing the servicelife by suppressing the microstructural change and deterioration of thematerial in a rolling fatigue process of the carburized components. In acase where there is a demand for high-strengthening by including Si, asuitable range of the Si content is 0.20% to 1.50%. In particular, in acase where case hardening steel is used as the material of a carburizedcomponent having a high level of rolling fatigue strength, it is moredesirable that the Si content is set to be in a range of 0.40% to 1.50%.

The effect of the inclusion of Si on the suppression of themicrostructural change and deterioration of the material in the rollingfatigue process of the bearing component or the rolling component isparticularly significant when the amount of retained austenite(so-called retained γ amount) in the structure after being subjected tocarburizing and quenching is 30% to 40%. In order to control theretained γ amount to be in this range, it is effective to performnitriding in a diffusion treatment after carburizing (so-calledcarbonitriding treatment). The nitriding treatment after the carburizingis appropriately performed under the condition that the nitrogenconcentration on the surface is in a range of 0.2% to 0.6%. It isdesirable that the carbon potential during carburizing in this case isset to be in a range of 0.9% to 1.3%.

(Mn: 0.30% to 1.80%)

Mn is an element effective in deoxidizing steel. In addition, Mn is anelement effective in imparting necessary strength and hardenability tosteel. When the Mn content is less than 0.30%, the effect is notsufficiently obtained. Therefore, the Mn content is set to 0.30% ormore, and is desirably 0.50% or more. On the other hand, when the Mncontent exceeds 1.80%, not only is the effect saturated, but also thecold forgeability deteriorates due to an increase in the hardness of thesteel. Therefore, it is necessary to set the Mn content to 1.80% orless, and desirably 1.20% or less. In a case where the cold forgeabilityof the steel is considered as being important, it is desirable to setthe Mn content to be in a range of 0.50% to 0.75%.

(P: 0.050% or Less)

P is an element which causes deterioration of cold forgeability byincreasing deformation resistance during cold forging and thus causingdeterioration of toughness. In addition, P is an element which causesdeterioration of fatigue strength by embrittling grain boundaries of acomponent after quenching and tempering. Therefore, it is desirable toreduce the P content as much as possible. However, when the P contentexceeds 0.050%, deterioration of the cold forgeability and fatiguestrength becomes significant, so that the P content is limited to 0.050%or less. A suitable range of the P content is 0.015% or less. The Pcontent may also be 0%.

(S: 0.003% to 0.020%)

S is an element that forms MnS in steel. Since MnS can be the origin ofbending fatigue fracture of a carburized component, it is necessary toprevent the formation of MnS. Therefore, the S content is set to 0.020%or less, and the relationship between the S content and the Ti contentis set to be in a range satisfying Expression (1). When the S content isin the above range, S in steel is present as Ti-based carbosulfide, andthus excellent fatigue properties are obtained after carburizing andquenching. The S content is more preferably 0.015% or less. On the otherhand, the Ti-based carbosulfide has the pinning effect that contributesto the prevention of the generation of coarse grain. In order to exhibitthe effect, it is necessary to set the S content to 0.003% or more. TheS content is preferably 0.005% or more.

Ti/S≥6.0   Expression (1)

(Ti in Expression (1) is the Ti content (mass %), and S in theExpression (1) is the S content (mass %))

(Cr: 0.40% to 2.00%)

Cr is an element effective in improving the strength and hardenabilityof steel. Furthermore, in a case where case hardening steel is used asthe material of a carburized component such as a bearing component and arolling component, Cr increases the retained γ amount after carburizingand quenching and suppresses the microstructural change anddeterioration of the material in a rolling fatigue process. Therefore,Cr is an element that contributes to an increase in the fatigue life ofthe carburized component. When the Cr content is less than 0.40%, theeffect is insufficient. Therefore, it is necessary to set the Cr contentto 0.40% or more. The Cr content is preferably 0.70% or more. On theother hand, when the Cr content exceeds 2.00%, the cold forgeabilitydeteriorates due to an increase in the hardness of the steel. Therefore,it is necessary to set the Cr content to 2.00% or less. The Cr contentis preferably 1.60% or less.

The effect of the inclusion of Cr on the suppression of themicrostructural change and deterioration of the material in the rollingfatigue process of the bearing component or the rolling component isparticularly significant when the retained y amount in the structureafter being subjected to carburizing and quenching is 30% to 40%. Inorder to control the retained γ amount to be in this range, it iseffective to perform a nitriding treatment after the carburizing underthe condition that the nitrogen concentration on the surface is in arange of 0.2% to 0.6%.

(Al: 0.005% to 0.050%)

Al is an element effective as a deoxidizing agent. When the Al contentis less than 0.005%, the effect is insufficient. Therefore, the Alcontent is set to 0.005% or more. The Al content is preferably 0.025% ormore. On the other hand, when the Al content exceeds 0.050%, a portionof AlN remains unsolubilized during heating before hot rolling performedat the time of manufacturing of case hardening steel and becomes aprecipitation site of precipitates of Ti (Ti and Nb in a case where Nbis contained). In this case, fine dispersion of the Ti-basedprecipitates (Ti-based precipitates and Nb carbonitride in the casewhere Nb is contained) is inhibited such that grains coarsen duringcarburizing. Therefore, it is necessary to set the Al content to 0.050%or less. The Al content is preferably 0.040% or less.

(Ti: 0.06% to 0.20%)

Ti is an element that forms fine Ti-based carbide and Ti-basedcarbosulfide such as TiC, TiCS, and Ti₄C₂S₂ in steel and is an elementeffective for achieving y grain refinement during carburizing. When theTi content is less than 0.06%, the effect is insufficient, and thus theTi content is set to 0.06% or more. On the other hand, when the Ticontent exceeds 0.20%, precipitation hardening by TiC significant occursand the cold workability deteriorates significantly. In addition, theformation of precipitates mainly containing TiN significantly occurs,and the rolling fatigue properties after carburizing and quenchingdeteriorate. Therefore, it is necessary to set the Ti content to 0.20%or less. The Ti content is preferably less than 0.15%.

When the case hardening steel according to this embodiment or a forgedmember obtained by forging the case hardening steel is subjected tocarburizing and quenching, a solid-soluted Ti reacts with carbon andnitrogen infiltrating during the carburizing such that a large amount offine TiC and TiN (hereinafter, sometimes referred to as “Ti(C, N)”)precipitates on the carburized layer. The Ti(C, N) contributes to theimprovement of rolling fatigue life in a carburized component such as abearing component or a rolling component obtained by performingcarburizing and quenching on the case hardening steel. Therefore, in acase where the bearing component or the rolling component, which demandsa high level of rolling fatigue life, is manufactured, it is effectiveto promote the precipitation of the Ti(C, N) by setting the carbonpotential during the carburizing to a higher level in a range of 0.9% to1.3% or by performing a so-called carbonitriding treatment. Thecarbonitriding treatment is a treatment in which the above-describedcarburizing and nitriding in a diffusion treatment after the carburizingare performed, and in the nitriding treatment, the condition that thenitrogen concentration on the surface is in a range of 0.2% to 0.6% isappropriate.

(Bi: 0.0001% to 0.0050%)

Bi is an important element in the case hardening steel according to thisembodiment. When a small amount of Bi is contained in the steel, sulfideis finely dispersed as the solidification structure of the steel (mainlydendrite structure) becomes refined. Furthermore, by including a smallamount of Bi in the steel, during the carburizing, growth and coarseningof precipitates such as Ti-based precipitates that suppress coarseningof grains can be suppressed. In order to obtain the above effect, it isnecessary to set the Bi content to 0.0001% or more. The Bi content ispreferably 0.0010% or more. On the other hand, when the Bi contentexceeds 0.0050%, the effect of refining the solidified structure issaturated and the hot workability of the steel deteriorates. This makesit difficult to perform the hot rolling at the time of manufacturing ofthe case hardening steel. For these reasons, the Bi content is set to0.0050% or less. The Bi content is preferably 0.0040% or less.

(N: 0.0060% or Less)

When N is bonded to Ti in steel, coarse TiN which hardly contributes tothe prevention of coarsening of grains is formed. TiN becomes aprecipitation site of Ti-based precipitates mainly containing TiC andTiCS, and NbC and NbN mainly containing NbC (hereinafter, sometimesreferred to as “Nb(C, N)”) and inhibits fine precipitation of Ti-basedprecipitates and Nb(C, N). In this case, the generation of coarse grainscannot be sufficiently suppressed. This adverse effect is particularlysignificant in a case where the N content exceeds 0.0060%. For the abovereasons, it is necessary to set the N content to 0.0060% or less. The Ncontent is preferably less than 0.0051%. The N content may also be 0%.

(O: 0.0025% or Less)

In high titanium steel (steel containing a large amount of Ti) such asthe case hardening steel according to this embodiment, O in the steelforms Ti-based oxide inclusions. Since the Ti-based oxide inclusionsbecome a precipitation site of TiC, when a large amount of the Ti-basedoxide inclusions are present in the steel, TiC coarsely precipitatesduring the hot rolling in the manufacturing of the case hardening steel.In this case, coarsening of grains during the carburizing cannot besuppressed. Therefore, it is desirable to reduce the O content as muchas possible. When the O content exceeds 0.0025%, the adverse effectbecomes significant. Therefore, it is necessary to limit the O contentto 0.0025% or less. A suitable range of the O content is 0.0020% orless. In a carburized component such as a bearing component or a rollingcomponent, the oxide inclusions become the origin of rolling fatiguefracture. Therefore, the lower the O content of the case hardeningsteel, the longer the rolling life of the carburized component.Therefore, in a case where the case hardening steel is used as thematerial of the carburized component such as the bearing component orthe rolling component, it is desirable to limit the O content to 0.0012%or less. The O content may also be 0%.

The case hardening steel according to this embodiment basically includesthe above-described elements and the remainder consisting of Fe andimpurities. However, instead of a portion of Fe, one or more elementsselected from the group consisting of Mo, Ni, V, B, and Nb may becontained in the following ranges in addition to the above-mentionedelements. However, these elements are not necessarily contained.Therefore, the lower limits thereof are 0%. In addition, even when theseelements are contained in amounts less than the following ranges, theelements do not impair the properties of the case hardening steel andare thus allowed.

In addition, the impurities are components incorporated from rawmaterials such as ore or scrap or from various environments in amanufacturing process when steel is industrially manufactured and areallowed in a range in which the steel is not adversely affected.

The chemical composition of the case hardening steel according to thisembodiment may further include one or more of Mo, Ni, V, B, and Nb asnecessary in the following ranges.

(Mo: 0.02% to 1.50%)

Mo is an element effective in improving the strength and hardenabilityof steel. Furthermore, Mo is an element effective in an increase in thefatigue life by increasing the retained γ amount in a bearing componentor a rolling component obtained after carburizing and suppressing themicrostructural change and deterioration of the material in a rollingfatigue process. In a case of obtaining these effects, it is preferableto set the Mo content to 0.02% or more. The Mo content is morepreferably 0.05% or more. However, when the Mo content exceeds 1.50%,machinability and cold forgeability deteriorate due to an increase inhardness. For the above reasons, even in the case where Mo is contained,the Mo content is set to be in a range of 1.50% or less. The Mo contentis preferably 0.50% or less.

The effect of the inclusion of Mo on the suppression of themicrostructural change and deterioration of the material in the rollingfatigue process of the bearing component or the rolling component isparticularly significant when the retained y amount in the structureafter being subjected to carburizing and quenching is 30% to 40%, likethe above-described effect of Cr.

(Ni: 0.10% to 3.50%)

Ni is an element effective in improving the strength and hardenabilityof steel. In a case of obtaining the effect, it is preferable to set theNi content to 0.10% or more. The Ni content is more preferably 0.20% ormore. On the other hand, when the Ni content exceeds 3.50%,machinability and cold forgeability deteriorate due to an increase inhardness. Therefore, even in a case where Ni is contained, the Nicontent is set to be in a range of 3.50% or less. The Ni content ispreferably 2.00% or less.

(V: 0.02% to 0.50%)

V is an element effective in improving the strength and hardenability ofsteel. In a case of obtaining the effect, it is preferable to set the Vcontent to 0.02% or more. However, when the V content exceeds 0.50%,machinability and cold forgeability deteriorate due to an increase inhardness. Therefore, even in a case where V is contained, the V contentis set to be in a range of 0.50% or less. The V content is preferably0.20% or less.

(B: 0.0002% to 0.0050%)

B is an element effective in improving the strength and hardenability ofsteel. In addition, B forms boron iron carbide in a steel bar or a wirerod in a cooling process after rolling and thus increases the growthrate of ferrite, thereby providing an effect of softening the rolledsteel. Furthermore, B improves the grain boundary strength of acarburized material and also has an effect of improving fatigue strengthand impact strength as a carburized component. In a case of obtainingthese effects, it is preferable to set the B content to 0.0002% or more.The B content is more preferably 0.0005% or more. However, when the Bcontent exceeds 0.0050%, the above-mentioned effects are saturated, andadverse effects such as deterioration of the impact strength areconcerned. Therefore, even in a case where B is contained, the B contentis set to be in a range of 0.0050% or less. The B content is preferably0.0030% or less.

(Nb: More Than 0% and Less Than 0.040%)

Nb is an element that is bonded to C and N in steel during carburizingto form Nb(C, N) and is thus effective in suppressing coarsening ofgrains. By including Nb, the effect of preventing coarse grains due toTi-based precipitates is further increased. This is because Nb issolid-solubilized in the Ti-based precipitates and thus suppressescoarsening of the Ti-based precipitates. The effect of the inclusion ofNb increases as the Nb content is increased. On the other hand, Nbcauses deterioration of machinability and cold forgeability, anddeterioration of carburizing properties. In particular, when the Nbcontent is 0.040% or more, the hardness of the material increases andthe machinability and the cold forgeability deteriorate. In addition, itis difficult for Nb carbonitride to be solid-solubilized by heatingduring hot rolling of the rolled material, and the number of grains ofthe Nb carbonitride finely precipitated decreases, leading todegradation of the coarse grain prevention properties. Therefore, evenin a case where Nb is contained, the Nb content is set to be less than0.040%. In a case where workability such as machinability and coldforgeability is considered as being important, an appropriate range ofthe Nb content is less than 0.030%. In a case where carburizingproperties are considered as being important in addition to workability,an appropriate range of the Nb content is less than 0.020%. Furthermore,in a case where carburizing properties are considered as beingparticularly important, an appropriate range of the Nb content is lessthan 0.010%.

Even when the Nb content is a small amount such as less than 0.030%,less than 0.020%, or less than 0.010%, Nb significantly improves thecoarse grain prevention properties compared to a case where Nb is notcontained. Therefore, in a case where it is desirable to obtain theabove-described effect, the Nb content may be more than 0%.

In order to achieve both the coarse grain prevention properties and theworkability, it is preferable to adjust the Nb content according to theTi content. Specifically, it is preferable to set the total content(Ti+Nb) of the Nb content and the Ti content to 0.07% to 0.20%. Inparticular, in a case where the case hardening steel is subjected tocarburizing at a high temperature or cold forging, a desirable range ofthe total content of the Nb content and the Ti content is more than0.091% and less than 0.17%.

Next, the structure (metallographic structure) of the case hardeningsteel according to this embodiment will be described.

(Maximum Diameter of Ti-Based Precipitates Predicted by Extreme ValueStatistics: 40 μm or Less)

In the case hardening steel according to this embodiment, in alongitudinal section, the maximum diameter of Ti-based precipitatespredicted by extreme value statistics under the condition that aninspection standard area is 100 mm², a number of inspections is 16visual fields, and an area where prediction is performed is 30,000 mm²is set to 40 μm or less.

One of the required properties of the carburized component obtained fromthe case hardening steel which is an object of this embodiment is theimprovement of contact fatigue strength such as rolling fatigueproperties and surface fatigue strength. When coarse Ti-basedprecipitates are present in the case hardening steel, the Ti-basedprecipitates become the origin of contact fatigue fracture in thecarburized component manufactured by performing carburizing andquenching the case hardening steel, resulting in deterioration of thefatigue properties.

When the maximum diameter of Ti-based precipitates predicted by extremevalue statistics under the condition that the inspection standard areais 100 mm², the number of inspections is 16 visual fields, and the areawhere prediction is performed is 30,000 mm² exceeds 40 μm, an adverseeffect of the Ti-based precipitates on the contact fatigue propertiesbecomes particularly significant. For the above reason, the maximumdiameter of the Ti-based precipitates predicted by extreme valuestatistics under the above condition is set to be 40 μm or less, and ispreferably 30 μm or less.

A method of measuring and predicting the maximum diameter ofprecipitates using extreme value statistics is based on the methoddescribed in pp. 233 to 239 of “Metal Fatigue: Effects of Small Defectsand Nonmetallic Inclusions” published by YOKENDO Ltd., on Mar. 8, 1993.In this embodiment, the two-dimensional inspection method in whichmaximum precipitates observed in a predetermined area (an area whereprediction is performed: 30,000 mm²) by two-dimensional inspection areestimated is used. Detailed measurement procedures will be described inExamples. The area where prediction is performed is set in considerationof a risk volume of a general component.

(Structure Fraction of Bainite: 30% or Less)

In the case hardening steel according to this embodiment, it ispreferable that the structure fraction (area ratio) of bainite is 30% orless. When a bainitic structure is incorporated in the case hardeningsteel, phase interface precipitation of Ti-based precipitates isdifficult, this causes the generation of coarse grains duringcarburizing. In addition, it is desirable that the bainitic structure inthe case hardening steel is small from the viewpoint of improving coldworkability. An adverse effect of the bainitic structure in the casehardening steel is particularly significant when the structure fractionof the bainite exceeds 30%. For the above reasons, it is preferable tolimit the structure fraction of the bainite to 30% or less. In a casewhere carburizing conditions regarding the prevention of coarse grainsduring carburizing are severe, such as in a case where the casehardening steel is subjected to high temperature carburizing, a suitablerange of the structure fraction of the bainite is 20% or less. Inaddition, in a case where carburizing conditions regarding theprevention of coarse grains during carburizing are more severe, such asin a case where the case hardening steel is subjected to cold forging, asuitable range of the structure fraction of the bainite is 10% or less.The bainitic structure may also be 0%. A structure other than thebainite is preferably a structure mainly containing ferrite andpearlite.

(Ferrite Grain Size: No. 8 to No. 11)

In the case hardening steel according to this embodiment, the grain sizenumber of ferrite contained in the metallographic structure ispreferably No. 8 to No. 11 specified in JIS G 0552. When ferrite grainsof the case hardening steel are excessively fine, austenite grains areexcessively refined during carburizing. When the austenite grains becomeexcessively fine, the driving force for grain growth increases andcoarse grains tend to be formed. In particular, when the ferrite grainsize exceeds No. 11 specified in JIS G 0552, this tendency becomessignificant. On the other hand, when the ferrite grain size is less thanNo. 8 specified in JIS G 0552, the ferrite is coarse, the ductilitydeteriorates, and thus cold forgeability also deteriorates. For theabove reasons, it is preferable to set the ferrite grain size number tobe in a range of No. 8 to No. 11 specified in JIS G 0552.

Since the case hardening steel according to this embodiment hasexcellent coarse grain prevention properties during carburizing, heattreatment strain due to carburizing and quenching can be suppressed. Inaddition, when the carburizing and quenching is performed, a carburizedcomponent having excellent fatigue properties is obtained. Even when thecase hardening steel according to this embodiment is subjected to hightemperature carburizing, the generation of coarse grains during thecarburizing can be suppressed. Therefore, by performing the hightemperature carburizing after forging, the carburizing time can bereduced. Furthermore, even for a carburized component in the related artin which switching from hot forging to cold forging is not performed dueto deterioration of dimensional accuracy caused by heat treatmentstrain, switching to cold forging can also be performed. Moreover,annealing performed in the related art to suppress heat treatment strainafter cold forging can be omitted.

A carburized component according to this embodiment includes the casehardening steel according to this embodiment. The carburized componentaccording to this embodiment is manufactured, for example, by a methodof forging the case hardening steel according to this embodiment,machining the forged steel into a predetermined shape, and performingcarburizing, quenching and tempering thereon. During the forging,machining, carburizing and quenching, the chemical composition and themaximum diameter of the Ti-based precipitates do not change. Therefore,the carburized component according to this embodiment has the samechemical composition and Ti-based precipitates as those of the casehardening steel according to this embodiment. However, since thecarburized component according to this embodiment is obtained throughcarburizing and quenching, the carburized component according to thisembodiment is different from the case hardening steel in that acarburizing and quenching layer is formed on the surface.

Next, a preferable manufacturing method of the case hardening steelaccording to this embodiment will be described in detail.

The manufacturing method described below is merely an example, and aslong as a case hardening steel satisfying the scope of this embodimentcan be obtained, the manufacturing method of the case hardening steelaccording to this embodiment is not limited to the followingmanufacturing conditions.

<Melting Process, Casting Process, and Blooming Process>

Steel having the above-mentioned chemical composition is melted (meltingprocess) according to a typical method such as a converter or anelectric furnace, and was cast into a bloom having the above-mentionedchemical composition (casting process). Thereafter, blooming isperformed as necessary (blooming process), thereby obtaining a rolledmaterial to be hot rolled into a wire rod or a steel bar. The size ofthe bloom, the cooling rate during solidification, and the bloomingconditions are not particularly limited.

<Heating Process, Hot Rolling Process, and Cooling Process>

Next, the rolled material having the above-described chemicalcomposition is heated under the following conditions, hot rolled into awire rod or a steel bar, and cooled, thereby obtaining case hardeningsteel.

In the manufacturing of the case hardening steel according to thisembodiment, the rolled material having the above-mentioned chemicalcomposition is heated at a temperature of 1150° C. or higher for aholding time of 10 minutes or longer (heating process), and the heatedrolled material is hot rolled into a wire rod or a steel bar (hotrolling process). During the hot rolling, when the heating temperatureis 1150° C. or higher and the holding time is 10 minutes or longer, theTi-based precipitates can be sufficiently solid-solubilized in a matrix.

When the heating temperature before the hot rolling is lower than 1150°C. and/or the holding time is shorter than 10 minutes, the Ti-basedprecipitates and AIN (in a case where Nb is contained, Ti-basedprecipitates, Nb precipitates, and AIN) cannot be sufficientlysolid-solubilized in the matrix. As a result, coarse Ti-basedprecipitates once formed in the casting process remain unsolubilized inthe steel after being hot rolled and cooled, and the Ti-basedprecipitates (Ti-based precipitates and Nb-based precipitates in thecase where Nb is contained) cannot be finely precipitated. Furthermore,coarsening of the Ti-based precipitates remaining unsolubilized in theheating process before the hot rolling proceeds due to Ostwald growth.As a result, coarse Ti-based precipitates and AIN (in the case where Nbis contained, coarse Ti-based precipitates, Nb-based precipitates, andAIN) are present in the steel after being hot rolled and cooled. In thiscase, the generation of coarse grains during carburizing cannot besuppressed. Therefore, during hot rolling, it is preferable to performheating at a temperature of 1150° C. or higher for a holding time of 10minutes or longer. A suitable range of the heating condition in the hotrolling is a holding time of 10 minutes or longer at a temperature of1180° C. or higher. It is not necessary to limit the upper limits of theheating temperature and the holding time. However, the upper limit ofthe heating temperature may be set to 1300° C. and the upper limit ofthe holding time may be set to 60 minutes in consideration of facilityrestrictions and productivity.

(Finish Temperature)

The finish temperature (finish rolling temperature) of the hot rollingis preferably set to 840° C. to 1000° C. By setting the finishtemperature of the hot rolling to be the above range, steel having aferrite grain size number of No. 8 to No. 11 specified in JIS G 0552 canbe obtained.

When the finish temperature is lower than 840° C., the ferrite grainsize becomes too fine, and coarse grains are likely to be generatedduring carburizing. On the other hand, when the finish temperatureexceeds 1000° C., the ferrite grains becomes coarse, and the hardness ofthe steel after being hot rolled and cooled increases, resulting indeterioration of cold forgeability. For the above reasons, it ispreferable to set the finish temperature of the hot rolling to 840° C.to 1000° C. In order to soften the steel, the finish temperature ispreferably 920° C. to 1000° C. On the other hand, in a case where thecase hardening steel is cold-forged and is subjected to annealing afterthe cold forging and before carburizing and quenching, the finishtemperature is preferably 840° C. to 920° C.

(Cooling Rate)

After the hot rolling, the steel is cooled (cooling process). In themanufacturing of the case hardening steel according to this embodiment,it is preferable to perform slow cooling at a cooling rate (averagecooling rate) of 1.00° C./s or less in a temperature range of 800° C. to500° C. after the hot rolling. By performing the cooling under the abovecooling conditions after the hot rolling, the time for which theTi-based precipitates pass a precipitation temperature range issufficiently secured, and dispersion of fine Ti-based precipitates ispromoted. In addition, by performing the cooling under the above coolingconditions, the structure fraction of bainite can be suppressed. As aresult, steel in which the structure fraction of bainite is 30% or lessand excellent coarse grain prevention properties are achieved duringcarburizing is obtained. When the cooling rate exceeds 1.00° C./s in theabove temperature range, there is concern that the structure fraction ofthe bainite may increase to more than 30%. In addition, at a highcooling rate in the above temperature range, the hardness of the steelafter being hot rolled and cooled increases, resulting in deteriorationof cold forgeability. Therefore, it is desirable to set the cooling ratein the above temperature range to be as low as possible. A suitablerange of the cooling rate in the above temperature range is 0.70 ° C./sor less.

In a case where air cooling is performed after the hot rolling, althoughdepending on the size of the case hardening steel, there is concern thatthe cooling rate in a range of 800° C. to 500° C. may exceed 1.00° C./s.Therefore, it is preferable to control the cooling rate to decrease. Asa method of reducing the cooling rate, for example, a method in which aheat insulation cover or a heat insulation cover with a heat source isinstalled in a rear stage of a hot rolling line and slow cooling of thesteel is performed after the hot rolling by the heat insulation covermay be employed.

<Spheroidizing Annealing Process>

Spheroidizing annealing may be performed on the steel ((wire rod orsteel bar): case hardening steel) after the cooling process asnecessary.

By performing the spheroidizing annealing, the steel is softened, andthus the load during cold forging can be reduced.

According to the above manufacturing method, the case hardening steelaccording to this embodiment is obtained. This case hardening steel issuitable as the material of a carburized component.

The carburized component according to this embodiment can bemanufactured by the method in which the case hardening steel accordingto this embodiment is forged, is machined into a predetermined shape,and is subjected to carburizing and quenching. In a case ofmanufacturing a carburized component using the case hardening steelaccording to this embodiment, the carburizing and quenching may beperformed after hot forging, or the carburizing and quenching may beperformed after cold forging.

In a case of manufacturing a carburized component by performingcarburizing and quenching after hot forging the case hardening steel,for example, the carburized component can be manufactured by hot forgingthe case hardening steel (wire rod or steel bar), performing a heattreatment such as normalizing as necessary, and performing machining,carburizing and quenching, tempering, and polishing as necessary.

Specifically, for example, the hot forging can be performed at a heatingtemperature of 1150° C. or higher.

In addition, although conditions during the carburizing and quenchingare not particularly limited, for example, high temperature carburizingmay be performed such that the carburizing temperature is in atemperature range of 950° C. to 1090° C. In order to improve the rollingfatigue life of the carburized component, the carbon potential duringthe carburizing may be set to be high in a range of 0.9% to 1.3%. Inaddition, a carbonitriding treatment in which nitriding is performed ina diffusion treatment after the carburizing may be performed. In thenitriding treatment after the carburizing, the condition that thenitrogen concentration on the surface is in a range of 0.2% to 0.6% isappropriate for improving the rolling fatigue life.

EXAMPLES

Hereinafter, the present invention will be specifically described indetail with reference to Examples.

Steel having the composition shown in Table 1 was melted in a converter,was continuously cast into a bloom, and was subjected to blooming asnecessary, thereby obtaining a 162 mm²(162 mm x 162 mm in a crosssection) rolled material (billet).

Subsequently, the billet was heated at a heating temperature shown inTable 2 for a holding time of 10 minutes or longer, was hot rolled at afinish temperature for hot rolling shown in Table 2, and was cooled at acooling rate shown in Table 2 in a temperature of 800° C. to 500° C.after the hot rolling, thereby manufacturing a steel bar with a diameterof 24 to 30 mm.

TABLE 1 Expression C Si Mn P S Al Ti (1) Bi No. (mass %) (mass %) (mass%) (mass %) (mass %) (mass %) (mass %) (Ti/S) (mass %) Steel of Present1 0.17 0.31 0.61 0.016 0.014 0.014 0.11 7.9 0.0018 Invention Steel ofPresent 2 0.22 0.16 0.95 0.019 0.013 0.037 0.15 11.6  0.0021 InventionSteel of Present 3 0.17 0.29 0.52 0.011 0.012 0.021 0.12 10.0  0.0016Invention Steel of Present 4 0.16 0.17 0.75 0.014 0.011 0.017 0.08 7.30.0014 Invention Steel of Present 5 0.15 0.16 0.92 0.013 0.009 0.0200.09 10.0  0.0018 Invention Steel of Present 6 0.17 0.38 0.96 0.0120.014 0.033 0.13 9.3 0.0017 Invention Steel of Present 7 0.23 0.38 0.940.017 0.016 0.032 0.15 9.4 0.0019 Invention Steel of Present 8 0.17 0.430.92 0.016 0.015 0.033 0.14 9.3 0.0017 Invention Steel of Present 9 0.190.20 0.76 0.014 0.012 0.034 0.18 15.0  0.0018 Invention Steel of Present10 0.25 0.16 0.51 0.017 0.008 0.040 0.17 21.0  0.0019 Invention Steel ofPresent 11 0.24 0.19 0.79 0.011 0.011 0.034 0.18 15.9  0.0017 InventionSteel of Present 12 0.16 0.29 0.62 0.019 0.012 0.016 0.19 15.9  0.0035Invention Comparative Steel 13 0.17 0.14 0.61 0.012 0.015 0.029 0.1912.6  — Comparative Steel 14 0.15 0.18 0.92 0.012 0.016 0.017 0.18 11.2 0.0200 Comparative Steel 15 0.17 0.10 0.67 0.016 0.051 0.026 0.08 1.60.0018 Comparative Steel 16 0.25 0.25 0.92 0.016 0.019 0.019 0.07 3.70.0021 Comparative Steel 17 0.21 0.15 0.84 0.011 0.012 0.021 — 0.2 —Comparative Steel 18 0.21 0.18 0.90 0.012 0.012 0.019 0.08 6.7 0.0021Comparative Steel 19 0.19 0.26 0.81 0.013 0.013 0.036 0.09 6.9 0.0016Comparative Steel 20 0.23 0.16 0.79 0.015 0.012 0.033 0.11 9.2 0.0017Comparative Steel 21 0.18 0.16 0.51 0.016 0.015 0.032 0.12 8.0 0.0014Steel of Present 22 0.15 0.81 0.58 0.017 0.014 0.025 0.10 7.1 0.0018Invention Steel of Present 23 0.15 1.10 0.97 0.018 0.013 0.031 0.1310.0  0.0014 Invention N Cr Mo Ni V B Nb O No. (mass %) (mass %) (mass%) (mass %) (mass %) (mass %) (mass %) (mass %) Steel of PresentInvention 1 0.0047 1.21 — — — — — 0.0018 Steel of Present Invention 20.0041 1.25 — — — — 0.030  0.0017 Steel of Present Invention 3 0.00461.06 — — — — — 0.0010 Steel of Present Invention 4 0.0051 1.14 0.16 — —— — 0.0018 Steel of Present Invention 5 0.0047 1.20 0.18 — — — — 0.0010Steel of Present Invention 6 0.0046 0.98 0.17 — — — — 0.0016 Steel ofPresent Invention 7 0.0056 1.10 0.15 — — — — 0.0011 Steel of PresentInvention 8 0.0049 1.25 — — — 0.0018 — 0.0014 Steel of Present Invention9 0.0044 1.01 — — 0.16 — — 0.0014 Steel of Present Invention 10 0.00381.24 — 0.30 — — — 0.0011 Steel of Present Invention 11 0.0047 1.17 — — —0.0021 — 0.0014 Steel of Present Invention 12 0.0042 1.24 — — — — —0.0018 Comparative Steel 13 0.0051 1.08 — — — — — 0.0011 ComparativeSteel 14 0.0049 1.03 — — — — — 0.0012 Comparative Steel 15 0.0041 0.99 —— — — — 0.0017 Comparative Steel 16 0.0048 1.15 — — — — — 0.0012Comparative Steel 17 0.0132 1.04 — — — — — 0.0013 Comparative Steel 180.0178 1.07 — — — — — 0.0010 Comparative Steel 19 0.0044 0.98 — — — — —0.0010 Comparative Steel 20 0.0072 1.02 — — — — — 0.0011 ComparativeSteel 21 0.0042 1.00 — — — — 0.0500 0.0016 Steel of Present Invention 220.0039 1.07 — — — — — 0.0010 Steel of Present Invention 23 0.0041 1.19 —— — — — 0.0017

TABLE 2 Maximum Quality of material of diameter of 1050° C. caiburizedTi-based Carburizing material Structure precipitates simulation RollingHot rolling conditions friction Ferrite in extreme Grain fatigueRotating Heating Finish Cooling of grain value Vickers coarsening γgrain life bending Steel temperature temperature rate bainite sizestatistics hardness temperature size (relative fatigue ClassificationNo. (° C.) (° C.) (° C./s) (%) number (μm) (HV) (° C.) number value)strength Steel of Present 1 1210 920 0.59 0 10 18 189 >1100 10 2.7 OKInvention Steel of Present 2 1210 930 0.58 0 9 38 173 >1100 9 2.7 OKInvention Steel of Present 3 1250 940 0.59 0 9 36 180 >1100 10 1.9 OKInvention Steel of Present 4 1230 930 0.54 0 10 29 187 >1100 10 2.4 OKInvention Steel of Present 5 1220 910 0.52 5 10 35 193 >1100 10 2.5 OKInvention Steel of Present 6 1150 940 0.51 0 9 28 164 >1100 10 2.2 OKInvention Steel of Present 7 1170 920 0.56 0 9 35 177 >1100 10 2.5 OKInvention Steel of Present 8 1160 930 0.48 0 8 17 207 >1100 10 2.7 OKInvention Steel of Present 9 1220 930 0.59 0 9 36 209 >1100 10 2.1 OKInvention Steel of Present 10 1240 930 0.58 5 9 26 180 >1100 10 2.0 OKInvention Steel of Present 11 1240 900 0.63 0 9 17 189 >1100 9 2.3 OKInvention Steel of Present 12 1220 920 0.59 0 9 18 170 >1100 10 2.2 OKInvention Comparative Steel 13 1180 910 0.54 0 10 34 164 1050 9 2.9 OKComparative Steel 14 1190 930 0.52 0 9 31 172 >1100 10 0.2 NGComparative Steel 15 1200 940 0.71 5 8 34 147 1000 3 0.4 NG ComparativeSteel 16 1240 930 0.60 0 8 28 217 >1100 9 1.0 NG Comparative Steel 171250 930 0.69 0 7 — 181 1050 3 1.0 NG Comparative Steel 18 1220 930 0.520 9 59 184 1050 3 0.7 NG Comparative Steel 19 1000 900 0.58 0 9 61 1951000 3 0.9 NG Comparative Steel 20 1220 920 0.59 0 10 45 201 >1100 9 0.8OK Comparative Steel 21 1220 930 0.55 0 10 62 161 1000 3 0.6 NG Steel ofPresent 22 1200 940 0.57 0 9 29 195 >1100 10 2.4 OK Invention Steel ofPresent 23 1210 920 0.58 0 9 31 225 >1100 9 2.5 OK Invention

The microstructure of each steel bar (case hardening steel) after beinghot rolled and cooled was observed, the structure was identified by thefollowing method, and the structure fraction of bainite was measured.

In addition, for each steel bar (case hardening steel), the ferritegrain size was measured according to the specification of JIS G 0552 andthe grain size number was examined.

For each steel bar (case hardening steel), the maximum diameter ofTi-based precipitates was predicted using extreme value statistics bythe following method.

For each steel bar (case hardening steel), the Vickers hardness wasmeasured as an index of cold workability by the following method.

In order to evaluate the coarse grain prevention properties, acarburizing simulation was performed under the following conditions.

Furthermore, the γ grain size number, the rolling fatigue life, and therotational bending fatigue strength were evaluated as the quality of thematerial after the carburizing by the following method.

The results are shown in Table 2.

“Structure Fraction of Bainite”

A sample was taken by cutting (traversing) each steel bar (casehardening steel) in a direction perpendicular to the axial direction.After burying the obtained sample in a resin, the cut surface (observedsection) was polished. The observed section after being polished wascorroded with Nital to expose and observe the microstructure, and abainitic structure in the microstructure was identified. Furthermore,the area ratio of the bainitic structure in the observed section wasobtained and was used as the structure fraction (%) of bainite.

A structure other than the bainite was ferrite, or ferrite and pearlite.

“Maximum Diameter of Ti-Based Precipitates”

Prediction of the maximum diameter of the Ti-based precipitates usingextreme value statistics was performed by the following method. Whetheror not the precipitates were based on Ti was determined by thedifference in contrast in an optical microscope. The validity of theidentification method based on the difference in contrast was confirmedin advance by a scanning electron microscope with an energy dispersiveX-ray spectrometer.

A test piece was taken from each steel bar (case hardening steel), and aregion of a 100 mm² inspection standard area (region of 10 mm×10 mm) wasprepared in advance for 16 visual fields in a longitudinal section ofthe steel bar. In addition, the maximum precipitate of Ti-basedprecipitates in each 100 mm² inspection standard area was detected andphotographed with an optical microscope at a magnification of1,000-fold. This was repeated 16 times (that is, the number ofinspections was 16 visual fields) for the visual field of each 100 mm²inspection standard area. From the obtained photograph, the diameter ofthe maximum precipitate in each inspection standard area was measured.In a case where the precipitate is elliptical, the geometric mean of themajor axis and the minor axis is obtained and taken as the diameter ofthe precipitate. 16 pieces of data of the 16 diameters of the maximumprecipitates obtained were plotted on extreme value probability paper bythe method described in pp. 233 to 239 of “Fatigue of Metals: Effects ofFine Defects and Inclusions” published by YOKENDO LTD. PUBLISHERS, amaximum precipitate distribution line (a linear function of maximumprecipitate diameter and extreme value statistics standardized variable)was obtained, and the maximum precipitate distribution line wassubjected to extrapolation such that the diameter of the maximumprecipitates in an area of 30,000 mm² where prediction was performed waspredicted.

“Vickers Hardness (HV)”

A sample was taken by cutting (traversing) each steel bar (casehardening steel) after rolling in the direction perpendicular to theaxial direction. After burying the obtained sample in a resin, the cutsurface (observed section) was polished. Regarding a portion at adiameter ¼ depth from the surface of the observed section after beingpolished, the Vickers hardness was measured five times in total under aload of 10 kg based on “Vickers hardness test—Test method” in JIS Z 2244(2009), and the average value was taken as the Vickers hardness. Whenthe Vickers hardness is 230 HV or less, excellent cold forgeability wasdetermined.

(Carburizing Simulation)

Each steel bar (case hardening steel) was subjected to spheroidizingannealing, and thereafter an upsetting test piece was prepared. Afterperforming upsetting at a rolling reduction of 50%, a carburizingsimulation was conducted under the following conditions.

In the carburizing simulation, the heating temperature was set to threetemperatures, 1000° C., 1050° C., and 1100° C., and in a case of any ofthe heating temperatures, heating was performed for five hours, followedby water cooling. The cut surface of each test piece after thecarburizing simulation was polished and then corroded, and prioraustenite grain sizes were observed to obtain a grain coarseningtemperature (coarse grains generation temperature). The measurement ofthe prior austenite grain sizes was performed according to JIS G 0551,about 10 visual fields were observed at a magnification of 400-fold, andthe generation of coarse grains was determined when even a single coarsegrain having a grain size number of No. 5 or less was present.

A grain coarsening temperature of higher than 1100° C. was determined asgood coarse grain prevention properties, and a grain coarseningtemperature of 1100° C. or lower was determined as inferior coarse grainprevention properties. The grain coarsening temperatures are shown inTable 2.

(Evaluation of Quality of Material After Carburizing)

Next, each steel bar (case hardening steel) was subjected to coldforging at a rolling reduction of 50%, a columnar rolling fatigue testpiece having a diameter of 12.2 mm and an Ono type rotating bending testpiece (with an R1.14 notch) having a parallel portion with a diameter of9 mm were prepared, and carburizing was performed under the condition offive hours and a carbon potential of 0.8% at 1050° C. The temperature ofa quenching oil was 130° C., and tempering was performed at 180° C. fortwo hours.

For each of the obtained carburizing and quenching material, the γ(austenite) grain size of the carburized layer was investigated by thefollowing method.

A samples was taken by cutting (traversing) the parallel portionsubjected to the Ono type rotating bending after the carburizing,quenching and tempering, in the direction perpendicular to the axialdirection. After burying the obtained sample in a resin, the cut surface(observed section) was polished. Corrosion was caused to exposeaustenite grains from the observed section after being polished, and theaustenite grain size was measured in a visual field centered on aposition at a depth of 200 μm from the surface according to thespecification of JIS G 0551.

For each of the carburizing and quenching material, the rolling fatigueproperties were evaluated using a point contact type rolling fatiguetesting machine (Hertz maximum contact stress 5884 MPa). As a measure offatigue life of the rolling fatigue properties, L10 life defined as“number of stress repetitions at which fatigue fracture occurs at acumulative failure probability of 10% obtained by plotting test resultson Weibull probability paper” was used. The rolling fatigue liferepresents a relative value of the L10 life of each material when theL10 life of Comparative Steel No. 17 as was set to 1.

For each of the carburizing and quenching material, the bending fatiguestrength was evaluated using an Ono type rotating bending fatiguetesting machine. Regarding the rotating bending fatigue strength, thematerial that withstood a stress of 550 MPa 10,000,000 times wasevaluated as “OK”, and the material that was fractured was evaluated as“NG”.

These results are collectively shown in Table 2.

As shown in Table 2, the grain coarsening temperatures of steels of thepresent invention (Nos. 1 to 12, 22, and 23) were higher than 1100° C.,they grain size of the carburized material heated at 1050° C. was alsoNo. 7 or higher in terms of grain size number, which means fine grains,and the rolling fatigue life and the result of the rotating bendingfatigue test were good.

On the other hand, since Comparative Steel No. 13 did not contain Bi,the grain coarsening temperature was lower than those of the steels ofthe present invention.

In addition, in Comparative Steel No. 14, since the Bi content exceededthe upper limit specified in the present invention, initial cracksassumed to have occurred during hot rolling were present, and thus therolling fatigue life and the result of the rotating bending fatigue testwere inferior to those of the steels of the present invention.

Comparative Steel No. 15 had a large S content and did not satisfyExpression (1). Therefore, fatigue fracture originated from MnS hadoccurred, and the rolling fatigue life of the result of the rotatingbending fatigue test were inferior to those of the steels of the presentinvention. In addition, in No. 15, precipitates of Ti-based carbonitrideeffective in preventing coarsening caused by the formation of a largeamount of Ti-based sulfide could not be sufficiently obtained, and thegrain coarsening temperature was lower than those of the steels of thepresent invention.

Since Comparative Steels Nos. 16 and 17 did not satisfy Expression (1),fatigue fracture originated from MnS occurred, and the rolling fatiguelife and the result of the rotating bending fatigue test were inferiorto those of the steels of the present invention.

In addition, in No. 17, precipitates of Ti-based carbonitride effectivein preventing coarsening could not be sufficiently obtained, and thegrain coarsening temperature was lower than those of the steels of thepresent invention.

In Comparative Steel No. 18, the N content was high, and coarse TiN wasformed. Therefore, the rolling fatigue properties and the rotatingbending fatigue properties were inferior to those of the steels of thepresent invention. Furthermore, in No. 18, the amount of precipitates ofTi-based carbonitride effective in preventing coarse grains was reduceddue to the formation of coarse TiN. Therefore, the grain coarseningtemperature was lower than those of the steels of the present invention.

In Comparative Steel No. 19, since the heating temperature beforerolling was low, coarse Ti-based precipitates formed in the castingprocess remained unsolubilized and became coarsened in the heatingprocess. Therefore, the rolling fatigue properties and the rotatingbending fatigue properties were interior to those of the steels of thepresent invention. In addition, as a result of the reduction in theamount of precipitates of Ti-based carbonitride effective in preventingcoarse grains, the grain coarsening temperature was inferior to those ofthe steels of the present invention.

Comparative Steel No. 20 had a large N content and thus had coarse TiNformed, so that the rolling fatigue properties were inferior to those ofthe steels of the present invention.

Since Comparative Steel No. 21 had a large Nb content, the carburizingproperties were degraded, and a sufficient carbon concentration couldnot be obtained. As a result, the strength was insufficient, and therolling fatigue life and the result of the rotating bending fatigue testwere inferior to those of the steels of the present invention.

INDUSTRIAL APPLICABILITY

The case hardening steel according to the present invention has apredetermined chemical composition, and the maximum diameter of Ti-basedprecipitates is controlled to be in a predetermined range, therebyachieving excellent coarse grain prevention properties duringcarburizing. Therefore, with the case hardening steel according to thepresent invention, heat treatment strain due to carburizing andquenching can be suppressed, and excellent fatigue properties areobtained after the carburizing and quenching. In addition, thecarburized component manufactured by performing carburizing andquenching on the case hardening steel of the present invention has lessheat treatment strain and has excellent fatigue properties.

With the manufacturing method of a case hardening steel according to thepresent invention, the case hardening steel which has excellent coarsegrain prevention properties during carburizing can be manufactured. Thiscase hardening steel can suppress heat treatment strain due tocarburizing and quenching and achieves excellent fatigue propertiesafter the carburizing and quenching.

Therefore, the industrial effect of the present invention is extremelyremarkable.

1. A case hardening steel comprising, as a chemical composition, by mass%, C: 0.10% to 0.30%, Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S: 0.003%to 0.020%, Cr: 0.40% to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to 0.20%,Bi: 0.0001% to 0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0% to0.50%, B: 0% to 0.0050%, Nb: 0% or more and less than 0.040%, P: limitedto 0.050% or less, N: limited to 0.0060% or less, O: limited to 0.0025%or less, and a remainder including an iron and impurities, wherein anExpression (1) is satisfied, and in a longitudinal section, a maximumdiameter of Ti-based precipitates predicted by extreme value statisticsunder a condition that an inspection standard area is 100 mm², a numberof inspections is 16 visual fields, and an area where a prediction isperformed is 30,000 mm², is 40 μm or less,Ti/S≥6.0   Expression (1) where Ti in the Expression (1) represents a Ticontent by mass %, and S in the Expression (1) represents a S content bymass %.
 2. The case hardening steel according to claim 1 comprising, asthe chemical composition, by mass %, one or more selected from the groupconsisting of Mo: 0.02% to 1.50%, Ni: 0.10% to 3.50%, V: 0.02% to 0.50%,B: 0.0002% to 0.0050%, and Nb: more than 0% and less than 0.040%.
 3. Thecase hardening steel according to claim 1, wherein a metallographicstructure contains a bainite, and a structure fraction of the bainite is30% or less.
 4. The case hardening steel according to claim 1, whereinthe metallographic structure contains a ferrite, and a grain size numberof the ferrite is No. 8 to No. 11 specified in JIS G
 0552. 5. Acarburized component comprising: the case hardening steel according toclaim
 1. 6. A manufacturing method of a case hardening steel comprising:a heating process of heating steel including, as a chemical composition,by mass, C: 0.10% to 0.30%, Si: 0.02% to 1.50%, Mn: 0.30% to 1.80%, S:0.003% to 0.020%, Cr: 0.40% to 2.00%, Al: 0.005% to 0.050%, Ti: 0.06% to0.20%, Bi: 0.0001% to 0.0050%, Mo: 0% to 1.50%, Ni: 0% to 3.50%, V: 0%to 0.50%, B: 0% to 0.0050%, Nb: 0% or more and less than 0.040%, P:limited to 0.050% or less, N: limited to 0.0060% or less, O: limited to0.0025% or less, and a remainder including an iron and impurities, andsatisfies an Expression (2), at a temperature of 1150° C. or higher fora holding time of ten minutes or longer; and a hot rolling process ofhot rolling the steel into a wire rod or a steel bar,Ti/S≥6.0   Expression (2) where Ti in the Expression (2) represents a Ticontent (mass %), and S in the Expression (2) represents a S content(mass %).
 7. The manufacturing method of a case hardening steelaccording to claim 6 comprising: a cooling process of slow cooling thewire rod or the steel bar at a cooling rate of 1.00° C./s or less in atemperature range of 800° C. to 500° C. after the hot rolling process.8. The manufacturing method of a case hardening steel according to claim6, wherein, in the hot rolling process, a finish temperature is set to840° C. to 1000° C.
 9. The case hardening steel according to claim 2,wherein a metallographic structure contains a bainite, and a structurefraction of the bainite is 30% or less.
 10. The case hardening steelaccording to claim 2, wherein the metallographic structure contains aferrite, and a grain size number of the ferrite is No. 8 to No. 11specified in JIS G
 0552. 11. The case hardening steel according to claim3, wherein the metallographic structure contains a ferrite, and a grainsize number of the ferrite is No. 8 to No. 11 specified in JIS G 0552.12. The case hardening steel according to claim 9, wherein themetallographic structure contains a ferrite, and a grain size number ofthe ferrite is No. 8 to No. 11 specified in JIS G
 0552. 13. A carburizedcomponent comprising: the case hardening steel according to claim
 2. 14.A carburized component comprising: the case hardening steel according toclaim
 3. 15. A carburized component comprising: the case hardening steelaccording to claim
 4. 16. A carburized component comprising: the casehardening steel according to claim
 9. 17. A carburized componentcomprising: the case hardening steel according to claim
 10. 18. Acarburized component comprising: the case hardening steel according toclaim
 11. 19. A carburized component comprising: the case hardeningsteel according to claim
 12. 20. The manufacturing method of a casehardening steel according to claim 7, wherein, in the hot rollingprocess, a finish temperature is set to 840° C. to 1000° C.